Objective: The objective of this study was to evaluate genetic and pharmacokinetic factors affecting the initial pharmacotherapeutic effect of paroxetine (PAX) in Japanese patients with panic disorder (PD).
Method: Plasma concentration of PAX was determined by high performance liquid chromatography. Serotonin transporter gene-linked polymorphic region (5-HTTLPR) variants were determined by polymerase chain reaction techniques. PD severity was assessed using the Panic and Agoraphobia Scale (PAS).
Results: Multiple regression analysis revealed that the plasma concentration of PAX, 5-HTTLPR genotype, and comorbid physical illness were significant factors affecting the initial pharmacotherapeutic effect of PAX in PD and indicated that these factors accounted for 52.4% (R(2) = 0.524) of the variability in the percent reduction in PAS score. The final model was described by the following equation (P = 0.001): percent reduction in PAS score (%) = 68.5 - 1.2 x [plasma concentration of PAX (ng/ml)] - 33.0 x (L/S = 1, S/S = 0) - 21.8 x (with comorbid physical illness = 1, without comorbid physical illness = 0).
Conclusion: The high plasma concentration of PAX, the L/S genotype of 5-HTTLPR, and comorbid physical illness might be associated with a poor response to the initial phase of pharmacotherapy of PD with PAX.